The present invention relates to a displaying device in which a pixel signal voltage is periodically level-inverted together with a common voltage.
In recent years, flat-panel displaying devices represented by liquid crystal displaying devices have been widespread because of their advantages including thin profile, light weight, and low power consumption. A typical liquid crystal displaying device has a structure in which a liquid crystal composition is held between an array substrate and a counter substrate. Each of the array and counter substrates has a dielectric and light-transmitting property. A liquid crystal cell is formed by filling the liquid crystal composition in the gap between the array and counter substrates. The array substrate has a matrix array of pixel electrodes, scanning lines formed along the rows of pixel electrodes, signal lines formed along the columns of pixel electrodes, and a first alignment film entirely covering the matrix array of pixel electrodes. The scanning lines are provided for respectively selecting the rows of the pixel electrodes, and the signal lines are provided for applying a pixel signal voltage to the pixel electrodes of a selected row. The counter substrate has a common electrode opposed against the matrix array of pixel electrodes, and a second alignment film entirely covering the common electrode. The first and second alignment films are provided for causing liquid crystal molecules within the liquid crystal cell to be in a twisted nematic (TN) alignment when no potential difference is present between the pixel electrodes and the common electrode. When light is incident on the liquid crystal layer from one substrate side through a polarizing plate, the light is rotated along the liquid crystal molecules which are herically twisted in the direction of thickness of the liquid crystal layer and guided to the other substrate at which it is selectively transmitted through another polarizing plate. When a potential difference is created between the pixel electrode and the common electrode, the liquid crystal molecules are tilted up, from a plane parallel to the substrate surface on which an image is displayed, by an angle proportional to the potential difference, thereby changing the light transmittance.
In an active matrix liquid crystal displaying device, a plurality of thin film transistors (TFTS) are formed near the intersections of scanning lines and signal lines and used as switching elements for selectively driving the corresponding pixel electrodes. The gate of each TFT is connected to one scanning line, the drain is connected to one signal line, and the source is connected to one pixel electrode. The TFT is turned on in response to rising of a scanning pulse from the scanning line and supplies a pixel signal voltage from the signal line to the pixel electrode. The pixel electrode and the common electrode constitute a liquid crystal capacitance CLC which is charged in accordance with the potential difference between the electrodes. This potential difference is maintained by the liquid crystal capacitance CLC even after the TFT is turned off in response to falling of the scanning pulse.
However, when an electric field is always applied in the same direction, materials other than the liquid crystal are concentrated on one electrode side to shorten the service life of the liquid crystal cell. As a conventionally known technique of solving this problem, the polarity of the pixel signal voltage is inverted every frame period using the potential of the common electrode as a reference. In some cases, polarity inversion of the pixel signal voltage is performed every horizontal scanning period in order to reduce flicker. Recently, for the purpose of reducing the amplitude of the pixel signal voltage necessary for the above-described polarity inversion, the common electrode potential is positively shifted by a common voltage applied from a common electrode driver. In this case, the pixel signal voltage VSIG is level-inverted using the center level of the amplitude (0V to +5V) as a reference, and a common voltage VCOM is level-inverted from one of a high level VCOMH (=+5.2V) and a low level VCOML (=-0.2V) to the other in synchronism with level inversion of the pixel signal voltage VSIG, as shown in FIG. 9.
Conventionally, the common electrode driver obtains two stable power source voltage levels equal to the high level VCOMH and the low level VCOML from a DC/DC converter. However, this DC/DC converter has a complex structure since it must also generate a stable power source voltage level used for obtaining the pixel signal voltage VSIG. This makes it difficult to manufacture the liquid crystal displaying device at a low cost.
When a displaying device of a large size is manufactured, the common electrode requires an area increased in proportion to the number of pixel electrodes. Since the common electrode is constituted by a conductive thin film of Indium Tin Oxide or the like having a relatively high resistance, an increase in the load on the common voltage VCOM of the above-mentioned amplitude which drives the common electrode cannot be prevented.